Semiconductor laser diodes find a wide variety of uses in such fields as optical communications, optical memories, and high speed printing.
The most common type of semiconductor laser is the double heterostructure. The double heterostructure comprises a pair of relatively wide bandgap cladding layers of opposite conductivity type and a relatively narrow bandgap active layer located between the cladding layers. Typically, the active layer is low-doped or intrinsic. Such double heterostructures are usually formed using the AlGaAs materials system wherein the cladding layers comprise Al.sub.x Ga.sub.1-x As, where x is about 0.25 to about 0.35 and the active layer comprises GaAs or Al.sub.y Ga.sub.1-y As where y&lt;0.1. The layers are deposited on a GaAs substrate. Alternatively, the InP-InGaAsP materials system may be used for double heterostructures.
When the p-i-n structure formed by the cladding layers and the active layer is forward biased, electrons and holes are injected into and trapped in the active layer where efficient radiative recombination takes place. The wavelength band of the emitted radiation is determined by the bandgap of the active layer. In many lasers the emitted radiation is amplified as it travels back and forth between partially reflecting crystal facets at each end of the laser diode. The radiation is guided by a dielectric waveguide. Transverse confinement is provided by the cladding layers. Lateral confinement is provided by any of the common index guiding structures such as ridges or V-grooves. Thus, the crystal facets define a Fabry-Perot waveguide resonator which supplies frequency selective feedback for radiation emitted in the active layer. When the unsaturated round trip optical gain exceeds losses due to mechanisms such as scattering, absorption, and facet transmission, laser oscillations at a number of selected frequencies result. A coherent beam is emitted by one or both end facets of the laser. The advantages of the double heterostructure laser are discussed in Botez, "Laser Diodes are Power-Packed", IEEE Spectrum, June, 1985, pp. 43-54.
There are several problems which result from the use of partially reflecting crystal facets of a semiconductor body to provide the feedback necessary to sustain laser oscillations.
First, the available output power of the diode laser may be limited by damage to the facets. If an AlGaAs diode laser emits continuous wave optical power densities in excess of 6 to 9 mW per square micrometer of emitting area, the internal laser power density becomes so high that chemical reactions occur at the partly reflective crystal facets from which the light emerges. This causes the facet region to gradually darken (become absorbing), apparently the effect of a layer of amorphous oxide formed by a photo-chemical reaction. Over time the laser's output power at constant current degrades. The use of a passivating dielectric layer applied immediately after cleaving to form the facets can reduce the rate of window darkening. The passivating layer is typically a half wavelength in thickness, leaving the reflectivity of the facet unchanged.
In addition, laser light is absorbed, rather than amplified, near the facets because of non-radiative recombination of carriers at the facets, where the semiconductor material is terminated. At high optical power densities (20 to 25 mW per square micrometer) heavy light absorption in conjunction with non-radiative recombination induces a thermal runaway process, causing the end facets to melt, thereby catastrophically damaging the laser. The output power limits imposed by the gradual or catastrophic degradation of the laser diode's crystal end facets are lower as a result of a standing wave which is set up in the laser waveguide between the end facets which, as indicated above, form a Fabry-Perot resonator. The standing wave has a higher optical power at its antinodes than would a traveling wave. An antinode typically occurs right at the facet. This enhances the power dependent failure mechanisms. One solution is the use of an antireflection coated facet at the output end to reduce the amplitude of the standing wave. However, this significantly increases the laser current threshold and is only partially effective.
Besides failing at high output powers, a second problem with Fabry-Perot resonators formed by crystal end facets, is that such resonators provide feedback over a broad frequency range. Because the active layer has gain over a relatively large frequency band, diode lasers incorporating such Fabry-Perot resonators have multifrequency output and are not really suitable for high bit rate optical communications, which require that the laser output be restricted to a very narrow frequency spectrum.
One effective approach to producing a double heterostructure semiconductor diode laser having a narrow frequency spectrum involves the use of a distributed periodic reflecting structure to supply feedback, instead of a Fabry-Perot resonator. In the distributed feedback laser, feedback occurs at only one frequency and this is the frequency of oscillation. The oscillation frequency is EQU f.sub.o =1/2V.sub.p /.LAMBDA.
where V.sub.p is the phase velocity of the radiation in the waveguide and .LAMBDA. is the period of the distributed periodic reflecting structure. For laser oscillations to be sustained, the frequency f.sub.o must fall within the bandwidth in which the active layer has net positive gain.
However, such distributed feedback lasers are difficult to amplitude modulate at high speed without introducing undesirable frequency modulation. As the laser pumping current is varied to turn the laser on and off or to vary the output power, the concentration of charge carriers in and near the active layer varies, thereby causing variations in the gain of the laser. Generally speaking, such changes in the gain are accompanied by small changes in the index of refraction (the well known Kramers-Kronig relation and plasma effects). These change the phase velocity of the light propagating in the waveguide. As indicated above, the frequency of oscillation in distributed feedback lasers is proportional to the phase velocity. Thus, the optical pulses produced by the laser, necessarily exhibit frequency shifts or chirping during the pulse, limiting the useful bit rate at which the laser can be modulated. In other words, variations in the pumping current to achieve amplitude modulation, result in a frequency modulated optical signal with an undesirably high FM to AM ratio.
In view of the above, it is an object of the present invention to provide a double heterostructure semiconductor laser whose power output is not significantly limited by the gradual or catastrophic failure of partially reflecting crystal end facets.
It is a further object of the invention to provide a double heterostructure semiconductor laser which has a very narrow output frequency spectrum and that can be directly current modulated with a relatively low FM to AM ratio.